Respiratory ventilators are used to provide mechanical ventilation to a subject. The ventilation is employed to assist, or in some cases replace, spontaneous breathing. This ventilation can be life-saving and is a mainstay of intensive care medicine as well as during anesthesia.
A respiratory ventilator comprises a compressible air reservoir, air and oxygen supplies, a set of valves and tubes. The air reservoir is compressed several times a minute to deliver a mixture of air, oxygen, and/or other gases to the patient. When the overpressure is released, the patient exhales passively due to the elasticity of the lungs. The oxygen content of the inspired gas can be set from 21 percent (ambient air) to 100 percent (pure oxygen). The pressure and flow characteristics of the ventilator can be set mechanically or electronically.
In continuous mandatory ventilation, breaths are delivered at preset intervals, regardless of patient effort. This mode is utilized most often in the paralyzed or apneic patient because it can enhance breathing if respiratory efforts are present. In assist-control ventilation, the ventilator delivers preset breaths in coordination with the respiratory effort of the patient. With each inspiratory effort, the ventilator delivers a full assisted tidal volume. In this case, spontaneous breathing independent of the ventilator between breaths is not allowed.
In synchronous intermittent mandatory ventilation, the ventilator delivers preset breaths in coordination with the respiratory effort of the patient. Spontaneous breathing is allowed between breaths. The synchronization attempts to limit the barotrauma, which may occur with IMV when a preset breath is delivered to a patient who is already maximally inhaled (breath stacking) or is forcefully exhaling.
The operational parameters of the ventilator should be set so that the characteristics of the ventilation delivered to the subject (such as the pressure and flow rates) are optimized for the subject's individual needs. However, it is often not possible to assess the subject's individual requirements. Moreover, it is often not possible to know how the operational parameters of the ventilator should be set in order to achieve an optimal ventilation.
The term “spirometry” refers to a common test of respiratory function that involves measuring the total volume of air inhaled into the lungs over a respiratory cycle. A spirometry test is usually carried out by having a subject inhale air through a tube connected to an air flow meter that measures the total volume of air inhaled during the inspiration phase of the respiratory cycle. At the termination of the inspiration phase the subject exhales through the tube. A curve is generated showing the air flow as a function of time. The curve is analyzed to obtain one or more respiratory parameters of the subject that are used to assess the state of the respiratory system.
U.S. Patent entitled, “Method and system for analyzing respiratory tract air flow”, filed on Feb. 4, 2004 and having the publication number 2005017791 discloses a method for carrying out spirometry in at least a portion of an individual's respiratory tract. A plurality of microphones is fixed onto a subject's back or chest over the portion of the respiratory tract, and respiratory tract sounds are recorded from the region over a time interval from t0 to t1. An average acoustic energy during the subinterval is determined at a plurality of locations x in the region. The term “acoustic energy” at a location is used herein to refer to a parameter indicative of or approximating the product of the pressure and the mass propagation velocity at that location. The total average acoustic energy, summed over the locations x is then correlated with the airflow in the portion of the respiratory tract. The airflow may be calculated, for example, as to the logarithm of the total acoustic energy. The process may then be repeated during the expiratory phase of the respiratory cycle. The airflow in the lungs as a function of time during the inspiratory phase obtained may be integrated from a time t0 to a time t to produce a total volume of air that has flowed into the airways from time t0 to t. The airflow at a time t may be plotted as a function of the total volume of air that has flowed into the airways from t0 to time t, to produce a spirometry curve for inspiration. The process may then be repeated for the expiratory phase of the respiratory cycle.